linux-kernel - Re: [BUG] kernel: rcu: a possible sleep-in-atomic-context bug in srcu_read

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Date:   Mon, 13 Aug 2018 05:42:32 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     dave@...olabs.net, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] kernel: rcu: a possible sleep-in-atomic-context bug in
 srcu_read_delay()

On Mon, Aug 13, 2018 at 05:26:49PM +0800, Jia-Ju Bai wrote:
> 
> 
> On 2018/8/13 12:18, Paul E. McKenney wrote:
> >On Mon, Aug 13, 2018 at 11:04:10AM +0800, Jia-Ju Bai wrote:
> >>The kernel may sleep with holding a spinlock.
> >>
> >>The function call paths (from bottom to top) in Linux-4.16 are:
> >>
> >>[FUNC] schedule_timeout_interruptible
> >>kernel/rcu/rcutorture.c, 523: schedule_timeout_interruptible in
> >>srcu_read_delay
> >>kernel/rcu/rcutorture.c, 1105: [FUNC_PTR]srcu_read_delay in
> >>rcu_torture_timer
> >>kernel/rcu/rcutorture.c, 1104: spin_lock in rcu_torture_timer
> >>
> >>Note that [FUNC_PTR] means a function pointer call is used.
> >>
> >>I do not find a good way to fix, so I only report.
> >>This is found by my static analysis tool (DSAC).
> >Interesting.  I would have expected to have gotten a "scheduling while
> >atomic" error message, which I do not recall seeing.  And I ran a great
> >deal of rcutorture on v4.16.
> >
> >So let's see...  As you say, the rcu_torture_timer() function does in
> >fact acquire rand_lock in 4.16 and 4.17, in which case sleeping would
> >indeed be illegal.  But let's take a look at srcu_read_delay():
> >
> >static void
> >srcu_read_delay(struct torture_random_state *rrsp, struct rt_read_seg *rtrsp)
> >{
> >	long delay;
> >	const long uspertick = 1000000 / HZ;
> >	const long longdelay = 10;
> >
> >	/* We want there to be long-running readers, but not all the time. */
> >
> >	delay = torture_random(rrsp) %
> >		(nrealreaders * 2 * longdelay * uspertick);
> >	if (!delay && in_task()) {
> >		schedule_timeout_interruptible(longdelay);
> >		rtrsp->rt_delay_jiffies = longdelay;
> >	} else {
> >		rcu_read_delay(rrsp, rtrsp);
> >	}
> >}
> >
> >The call to schedule_timeout_interruptible() cannot happen unless the
> >in_task() macro returns true, which it won't if the SOFTIRQ_OFFSET bit
> >is set:
> >
> >#define in_task()		(!(preempt_count() & \
> >				   (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
> >
> >And the SOFTIRQ_OFFSET bit will be set if srcu_read_delay()
> >is invoked from a timer handler, which is the case for the
> >call from rcu_torture_timer().  So if that lock is held,
> >schedule_timeout_interruptible() won't ever be invoked.
> 
> Thanks for your reply :)
> My tool does not track this bit...
> Sorry for this false report.

Not a problem, a few false positives are to be expected.  And it looks
like you have some work to do on your tool -- which is good, because I
would not want you to be bored.  ;-)

							Thanx, Paul